Neutrinos and the Speed of Light — A Primer on the CERN Study

Cross section of the CNGS experiment through the Earth. (Image: CERN)

Recently, a group of physicists have been working to measure the neutrinos generated from a particle accelerator at CERN. This group discovered neutrinos arriving faster than would have been expected and they appear to be traveling faster than the speed of light itself, but they draw no definitive conclusions. This has been widely reported as being the end of Relativity, but this is not the case at all. Let’s take a look at what is going on in the experiment and what was reported in the journal article.

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First, it might help the reader to gain an understanding of the neutrino. Neutrinos are interesting little neutral particles that have almost zero mass. Due to their nature, they can pass through matter without being absorbed. There are three known types of neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino. The experiment in the journal article is referred to as CERN neutrinos to Gran Sasso, or CNGS. The CNGS team is searching for a phenomenon known as neutrino oscillation where muon neutrinos may change into tau neutrinos. A secondary goal of the experiment is to measure neutrino velocity to a great accuracy.

In the experiment, neutrinos are generated at the Super Proton Synchrotron (SPS) particle accelerator at the CERN LHC complex in Geneva and further accelerated down a 1 km beam line toward the Gran Sasso National Laboratory in Italy. At Gran Sasso, a detector instrument called OPERA measures the neutrinos. The distance from CERN to Gran Sasso is 732 km straight through the Earth, traveling up to 11.4 km below the Earth’s surface. Remember, neutrinos don’t interact with matter so the Earth is invisible to the tiny particles.

The distance between the two systems is known to within 20 cm. Time is also measured with extreme precision utilizing GPS timing signals and a cesium atomic clock. The GPS used in timing also allows the team to track any small movements in the Earth itself. This even allowed consideration of the effect of the L’Aquila Earthquake that moved the OPERA detector 7 cm. Due to the nature of the experiment, the time is not calculated with a simple, stopwatch style, start to finish measurement. It instead relies on measurements and comparisons of probability distribution functions at the source and the detector. In other words, there is a lot of math involved. In addition to understanding the timing and position variations in the experiment, the physicists also took into account many other variables, such as day versus night and seasonal changes. The sensitivity of this experiment is roughly an order of magnitude better than previous experiments.

The speed of neutrinos is measured and compared to the speed of light by subtracting the expected time for light to travel the distance from the time for the neutrinos to travel the same distance. One would normally expect this to be zero for neutrinos traveling at the speed of light or negative for any value below the speed of light. The case presented in the article shows a positive value of 60.7 nanoseconds with statistical and systematic errors providing not nearly enough potential difference to account for the positive value. This value has six-sigma significance. This is, obviously, a stunning finding.

The final paragraph is what appears to be overlooked all too often in the reporting on this finding:

Despite the large significance of the measurement reported here and the stability of the analysis, the potential great impact of the results motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results.

This is an important paragraph. This is the group of physicists, together, stating that they don’t know how they came to a result that shows neutrinos apparently exceeding the speed of light. They are not drawing any conclusions in this article and are simply providing the finding and the methods used to obtain the finding. They are trying to find where there could be errors in their measurements. They do not claim that the neutrinos are actually exceeding the speed of light, only that the measurements to date show something unexpected. They are reaching out to the high-energy physics community to improve the experiment and data analysis. They are not looking to fundamentally change physics but to ensure that they are producing sound data. We may find that nothing comes of this. We may find that there is an effect known in physics that accounts for the difference. We may find that neutrinos are capable of moving slightly faster than the speed of light. It is simply too early to make definitive, wide-reaching conclusions.

The conclusion that can be drawn from this article is that a group of experimenters found an unexpected result using some of the most amazing and precise instruments and techniques ever created. No matter what is found to be the actual cause of this 60.7 nanosecond variation, the conclusion you can draw is that it is an amazing time in history where such measurements can be made and an exciting time to be a practitioner or admirer of science. Imagine the findings that will be made by the next couple generations of scientists who are sitting in elementary classrooms right now and just learning that a rainbow is the spectrum of sunlight. Einstein wouldn’t be disappointed by these findings; he would be intrigued and proud to see the legacy of great science continuing forward.

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